1. Technical Field
The present invention relates to signal processing, and more particularly to two-stage precoding and user grouping for large scale multiple-input multiple-output (MIMO) systems.
2. Description of the Related Art
Most of the existing work directed to large scale MIMO (multi-input-multi-output) systems considers a Time Division Duplex (TDD) mode. However, there are as many FDD (Frequency Division Duplex) systems as TDD systems in real-world deployments. To equip a large amount of antennas in the base station (BS) of a FDD system, a two-stage precoding scheme has been proposed. In particular, all the users in the system are divided into several groups. The first stage precoding is designed to suppress the interferences among different groups. The second stage precoding is designed to suppress the interferences among different users within each group. Given the precoding methods, how to group users has a direct impact on the throughput performance. Fixing the number of groups, the existing grouping method may group too many users in some particular groups and group none or too few users to the other groups. Also, with groups being formed, scheduling which users of each group are to transmit and which users are to remain silent also has big impacts on the throughput. As is evident, such an approach is not a sufficient way to utilize the precious spectrum resources.